In the field of rubber industry, especially, automobile components are desired to have advanced function and performance. A frictional power transmission belt is one of rubber products used in such automobile components, and the frictional power transmission belt is widely used for, for example, power transmission of auxiliary drive of automobile air compressors, alternators and the like.
For example, a V-ribbed belt having ribs provided in a belt longitudinal direction is known as the belt of this kind. In recent years, a V-ribbed belt having silence (sounding resistance during drying and water pouring) or durability (abrasion resistance), particularly having both performances, is required as the V-ribbed belt. Furthermore, when water penetrates between a frictional power transmission surface having low water wettability and pulleys, water penetration condition becomes uneven between a belt and pulleys, and stick-slip sound is easy to be generated. For this reason, the improvement in sounding resistance when exposed to water is also required.
In response to this requirement, the means of covering a frictional power transmission surface with a fabric formed of fibers is known. In a V-ribbed belt in which rib portions are formed by a mold, the means of covering a frictional power transmission surface with a fabric is preferred for the reason that the fabric can be simultaneously fixed to the frictional power transmission surface during molding. Woven fabric made by weaving or knitted fabric made by knitting is used as the fabric. The knitted fabric has advantages that it is provided with flexibility to easily place along a frictional power transmission surface having concavo-convex formed due to a plurality of rib portions and it can easily secure stretchability of flowing up deformation of a belt body and stretching.
Japanese Patent No. 4942767 (Patent Document 1) discloses a V-ribbed belt having a layer of a warp knitted fabric containing two different kinds of yarns. This warp knitted fabric is constituted of a first yarn (e.g., polyamide) comprising filaments having a module of more than 5 N/1000 dtex at 5% extension and a second yarn (e.g., polyurethane) comprising filaments having a module of less than 2 cN/1000 dtex at 5% extension, and is characterized in that each of the first yarn and the second yarn forms a condensed network structure and the yarns are not mutually twined. For this reason, it is described that a V-ribbed belt can be effectively formed by elastic warp knitted fabric, whose extension does not depend on the construction of the knitted fabric, long-term durability can be improved, and noise generation can be suppressed. This document further describes that in the case where the knitting direction of the warp knitted fabric is arranged in a belt direction, only quite short pieces of yarn extend in the belt direction, the warp knitted fabric is dynamically stabilized, and polyamide fibers positioned outside efficiently protect a network structure from abrasion. It is further described that a flock layer such as cotton is formed on the surface of the warp knitted fabric in order to prevent generation of noise under the influence of a large amount of water.
However, in this V-ribbed belt, the yarns constituting the warp knitted fabric are the combination of polyamide or polyester and polyurethane, and water absorption is low. Therefore, water screen cannot be removed when exposed to water, and there is a concern of generation of sound. Furthermore, the relationship between a belt direction and a knitting direction of a warp knitted fabric is described, but specific data are not shown. It is assumed that the improvement in abrasion resistance greatly depends on the effect according to the polyamide fibers having excellent strength and stretchability, positioned outside (frictional power transmission surface) the knitted fabric. Furthermore, in the case where a flock layer is formed on the surface of the knitted fabric, the flock layer is easy to fall off during traveling.
Japanese Patent No. 5337795 (Patent Document 2) discloses, as a first embodiment, a V-ribbed belt in which the surface of ribs is covered with a fabric stretchable in predetermined two directions and containing cellulose fibers. It is described that in this embodiment, a weft knitted fabric showing satisfactory extensibility in two directions may be used as the fabric. It is further described that in this embodiment, because the surface of ribs is covered with a fabric and the fabric contains cellulose (cotton), abnormal noise generation can be suppressed at the time of slipping or being exposed to water. As a second embodiment, a V-ribbed belt in which a rubber is penetrated from the fabric covering the surface of ribs and is exposed on a frictional power transmission surface is disclosed. It is described that the characteristics (friction coefficient and abrasion resistance) of the rib surface can be controlled by controlling the amount of rubber penetrated onto the rib surface.
However, the V-ribbed belt of the first embodiment contains a large amount of cellulose (cotton). Therefore, abrasion resistance of the belt surface is decreased. Furthermore, in the V-ribbed belt of the second embodiment, in the case where the rib rubber layer penetrates on the rib surface, the area at which the fabric (weft knitted fabric) exposes on the surface thereof is decreased. Therefore, a friction coefficient in a dry state is increased. Furthermore, even though the fabric contains cellulosic fibers and shows water absorbability, it cannot absorb the water screen sufficiently on the whole frictional power transmission surface due to the presence of the rubber layer, the part in a dry state (DRY) and the part in a water-exposed state (WET) are present together. As a result, the difference in the friction coefficient of those parts is increased, and therefore, there is a concern of sound generation. Furthermore, rubber is penetrated from the fabric in the second embodiment, but even in the first embodiment, the fabric is made to follow on the rib surface by utilizing its stretchability, and therefore, the rubber is liable to partially penetrate the fabric to be exposed on the surface.
Japanese Patent No. 5302074 (Patent Document 3) discloses a V-ribbed belt covered with a rib-side knit reinforcement cloth in which the rate of elongation when load of 50 N is applied to a strip test piece having a width of 3 cm is from 100 to 500% in a belt length direction and the rate of elongation in a belt width direction is from 150 to 500%. This V-ribbed belt is characterized in that a rib surface rubber layer is provided between a rib of the belt body and the rib-side knit reinforcement cloth, and the rib surface rubber layer is partially oozed out of stitches of the rib-side knit reinforcement cloth to be exposed on a pulley contact surface. It is described that a pulley contact part can be maintained in a state of low friction coefficient without deteriorating heat resistance, and excellent abnormal sound generation suppressing effect can be obtained during belt traveling. Furthermore, this document describes that the increase of a friction coefficient due to follow-up properties of the knit reinforcement cloth into a rib shape and due to oozing of a rubber can be controlled by specifying the rate of elongation of the knit reinforcement cloth in a longitudinal direction and width direction of a belt to a specific range.
However, this V-ribbed belt is that a rubber layer is exposed on a pulley contact part (rib surface). Therefore, a friction coefficient is greatly decreased when exposed to water, and there is a concern of sound generation. Furthermore, in the knit (knitted fabric), generally extension and strength greatly differ depending on a knitting direction. It is not clearly described in this document as to a knitted texture of a knit reinforcing cloth and as to whether a knitting direction of a knit corresponds to either direction of a belt longitudinal direction or width direction.